In recent years, a group-III nitride semiconductor has received a great deal of attention as a semiconductor material used for a light-emitting device such as a light-emitting diode or a laser diode. Examples of the group-III nitride semiconductor are GaN and InGaN. This nitride semiconductor has a band gap energy corresponding to a wide wavelength range from infrared light to ultraviolet light and is a promising material as a light-emitting diode (LED) such as a blue or green LED and a semiconductor laser material whose oscillation wavelength ranges from the ultraviolet light to the infrared light. A white LED which is configurated with a combination of a blue LED that consists of a nitride semiconductor and a yellow fluorescent material has been practiced as for the energy-saving lighting and widely commercially available. For example, a semiconductor laser which consists of a nitride semiconductor and wavelength of which is 400 to 410 nm has been used for read and write of a commercially available high-density DVD (Digital Versatile Disk).
In such a device, a thin film semiconductor with excellent crystallinity is indispensable. In general, a semiconductor device is constructed with semiconductor multi-layers formed on a single crystalline substrate. These semiconductor multi-layers are the same kind of materials as the substrate material. This structure makes it possible to grow high-quality single-crystalline thin films which is almost defect-free. Among the defects, a defect particularly noticeable is a dislocation. In an optical device, the dislocation density greatly influences the device characteristics such as the luminous efficiency and the device lifetime. For example, in a semiconductor laser diode constructed with InP and its related materials for the optical communications systems, the device lifetime of 100,000 hours which is the requirement from the system can be realized by suppressing the dislocation density less than 103/cm2 for the first time.
However, GaN as the main material of the nitride semiconductors has the equilibrium vapor pressure of nitrogen (N) between the gas phase and the solid phase is higher by several orders of magnitude than the equilibrium vapor pressures of conventionally used III-V semiconductor materials such as phosphorus (P) of InP. For this reason, it is difficult to fabricate a GaN wafer, and its cost is very high. Currently sapphire substrates have been used in commercially available white LEDs and blue traffic lights. A lattice mismatch between GaN and sapphire is 13.8%, and threading dislocations with the density of 108-9/cm2 exist in GaN. For this reason, the efficacy of the white LED is currently as low as 180 lm/w which is twice or more of the fluorescent light.
In an ultraviolet LED having a wavelength of 260 nm and expected as a sterilization lamp, the luminous efficiency is improved along with a decrease in dislocation density (see Non-Patent Literature 1). However, since a band gap difference between a light-emitting layer and a carrier injection layer (clad layer) cannot be increased due to the limitation of a band-gap energy unique to a material, the luminous efficiency almost reaches its limit.
Nitride semiconductors are expected to provide high-performance transistors due to the physical properties of nitride semiconductors. For a so-called lateral transistor such as a high electron mobility transistor (HEMT) in which carriers are travelled parallel to a substrate surface, the threading dislocation lowers the mobility of electrons. In a vertical transistor in which carriers are travelled perpendicular to a substrate surface, an operation at a high breakdown voltage and a high power is expected. Since the carriers are travelled parallel to the threading dislocation, the vertical transistor is influenced by the threading dislocation more than the lateral transistor.
In view of the above situations, one solution for a substrate is to use a GaN substrate. Currently, a GaN substrate called a “free-standing substrate” is commercially available. Its price is about 200,000 yen per 2-inch diameter. This substrate is produced through many steps including GaN growth on a single-crystal substrate such as GaAs or sapphire, substrate removal, GaN cutting, and polishing. As for an ultraviolet LED, the use of this free-standing GaN substrate can reduce the dislocation density in the LED structure and implements the improvement of the luminous efficiency by one or more orders of magnitude (see Non-Patent Literature 2).